Effect of substrate hydrophobicity and electrolytes upon the micellar hydrolysis of p-nitrophenyl esters (original) (raw)
Related papers
2007
The pseudo-first order rate constant for the reaction of p-nitrophenyl acetate by benzohydroxamate ion was determined in water-ethylene glycol mixtures using cationic micellar solutions of alkyldimethyl and diethyl ethanol ammonium bromides. From the conductivity data of these surfactants in ethylene glycol, the critical micelle concentration (cmc), the micellar ionization degree (a), and Gibbs free energy of the micellization (DG 0 m ), have been determined. Addition of ethylene glycol in the micellar solutions results in an increase in the cmc and in an increase in micellar ionization degree. The dependence of observed rate constants were explained considering the structure of interfacial region, micellar aggregation number, water content in the micellar interfacial region and the equilibrium binding constants for the p-nitrophenyl acetate and benzohydroxamate ion to the micellar aggregates. The kinetic micellar effect was quantitatively explained by the pseudophase model.
Journal of Physical Organic Chemistry, 1991
Two amphiphilic oximes, 10-phenyl-10-hydroxyiminodecanoic acid (oxime 11) and 4-(9-carboxynonanyl)-l-(9-carhoxy-1-hydroxyimino nonany1)benzene (oxime 111) were synthesized. The pK. values of oximes I1 and I11 and acetophenoxime (oxime I) and the rates of oximolysis of p-nitrophenyl acetate (NPA) and p-nitrophenyl octanoate (NPO) were determined in the presence and absence of micellar hexadecyltrimethylammonium bromide (CTAB). The rates of oximolysis increased by up to 3 x 104-fold in the presence of CTAB. Quantitative analysis of micellar effects, using an ion-exchange pseudo-phase model, allowed the determination of the second-order rate constants for the reactions of oximes 1-111 with NPA and NPO in the micellar pseudo-phase. The calculated rate constants in the micellar pseudo-phase were lower than those in water, demonstrating that the rate enhancements were due to substrate concentration in the micelles. Comparison of the rate constants in micelles and water suggests that the sites of reaction of oximes 1-111 with NPO and NPA are similar to those in hulk aqueous solution. Micellar incorporation of the hydrophobic oximes I1 and I11 does not lead to a major change in the nucleophilicity of the oximate anion.
Journal of Colloid and Interface Science, 2001
The reaction of dehydrobromination of 2-(p-nitrophenyl)ethyl bromide with hydroxide ions has been studied in aqueous micellar solutions of N-tetradecyl-N,N-dimethyl-3-ammonio-1propanesulfonate, SB3-14. The kinetic effects of added salts (NaF, NaCl, NaBr, and NaNO 3) on the reaction rate in SB3-14 aqueous micellar solutions have also been studied. They were rationalized by considering the binding of the anions, which come from the salt, to the sulfobetaine micelles and their competition with the reactive hydroxide ions for the micellar surface. The equilibrium binding constant of the 2-(p-nitrophenyl)ethyl bromide to the sulfobetaine micelles was estimated by recording the changes in the spectra of the organic substrate when the SB3-14 concentration in the micellar medium changed. This value was in agreement with that obtained from fitting of kinetic data. The second-order rate constant in the micellar pseudophase revealed that the reaction is faster in SB3-14 micelles than in water. This acceleration seems independent of the presence of added salts and can be explained by considering that SB3-14 micelles favor reactions in which charge is delocalized in the transition state.
Micellar effect upon the rate of alkaline hydrolysis of carboxylic and carbonate esters
Journal of Saudi Chemical Society
The alkaline hydrolysis of carboxylate (1-naphthylbutyrate) and carbonate esters (2-(methylsulfonyl)-ethyl-4-nitrophenylcarbonate) in the presence different surfactants has been studied. The rate of hydrolysis of these esters was determined under pseudo first order condition in which concentration of NaOH was kept in large excess over the [ester]. The cationic micelles of cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium sulfate ((CTA)2SO4) enhanced the rate of hydrolysis of esters to a maximum value and, thereafter, the increasing concentration of surfactant decreased the reaction rate. The anionic micelles of sodium dodecyl sulfate (SDS) inhibited the rate of the hydrolysis. The reaction proceeds through the attack of OH- ions on the carbonyl carbon forming tetrahedral intermediate. The tetrahedral intermediate is unstable and collapses immediately to yield respective acid and alcohol. The micelles influence the stability of tetrahedral intermediate, in turn, alter...
Effect of micelles on the acid hydrolysis of N-phenylbenzohydroxamic acid
Reaction Kinetics and Catalysis Letters, 1997
The effect of cationic (cetylpyridinium bromide and cetylpyridinium chloride) anionic (sodium dodecyl sulfate and lithium dodecyl sulfate) and non-ionic (Brij-35 and Triton-X-100) micelles on the acid-catalyzed hydrolysis of Nphenylbenzohydroxamic acid in 20 vol.% dioxane medium, has been investigated. The kinetic results are explained by both pseudo-phase and Piszkiewicz cooperativity models.
International Journal of Chemical Kinetics, 2011
The reactions of p-nitrophenyl acetate (PNPA) with a series of monopyridinium oximes, viz. 2-PAM (2-hydroxyiminomethyl-1-methylpyridinium iodide), 3-PAM (3-hydroxyiminomethyl-1-methylpyridinium iodide), and 4-PAM (4-hydroxyiminomethyl-1methylpyridinium iodide) have been studied in the presence of cationic surfactants of same hydrophobic chain length (C 16 ) within the concentration range of 0.5-6.0 mM at pH 8.0 under the pseudo-first-order condition. The observed rate constant (k obs ) increases with increasing surfactant concentration culminating into a maximum, and this has been analyzed in detail following the concepts of micellar catalysis. The structure-activity relationship of the investigated oximes has been discussed, and 2-PAM was found to be the most reactive among all the three investigated oximes for the cleavage of PNPA. Esterolytic decomposition of pnitrophenyldiphenyl phosphate with oximate ions ( CH NO − ) was followed in cetyltrimethylammonium bromide micelles at pH 9.0, and 4-PAM was the most reactive oxime for the micellar hydrolysis of phosphate ester. The apparent acid dissociation constants (pK a ) of the investigated oximes have been determined spectrophotometrically. C 2011 Wiley Periodicals, Inc. Int J Chem
Micellar effects on Aromatic Esters Hydrolysis
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2014
Highlights Cationic surfactants catalyze the alkaline hydrolysis of aromatic esters. Variation of counter-ions association degree to micelles explains experimental data. Adsorption equilibrium constants of esters do not depend on micelle counter-ion.
Ion exchange in micellar solutions. 4. "Buffered" systems
The Journal of Physical Chemistry, 1980
The alkaline hydrolysis of the positive micelle-excluded N-methyl-Ccyano-and N-methyl-2-cyanopyridinium ions (4-MCP and 2-MCP) and of the efficiently micelle-incorporated neutral substrate p-nitrophenyl octanoate (NPO) has been investigated in buffered micellar solutions of N-hexadecyl-Nflfl-trimethylammonium bromide (CTAB). The observed rate constants for hydrolysis of 4-MCP (pH 9.80) and 2-MCP (pH 9.50) are independent of CTAB, clearly demonstrating that the intermicellar pH is maintained constant by the buffer system employed (0.020 M borate) up to at least 0.10 M detergent. In contrast, the observed rate constants for hydrolysis of NPO (pH 9.50 and 9.80) under the same conditions vary with CTAB above the cmc, reflecting the variation of the local concentration of bound OH-[ m b ] in the micellar pseudophase with detergent Concentration. These results unambiguously demonstrate that buffering of the intermicellar aqueous phase does not imply unique "buffering" of the micellar pseudophase. The kinetic results for NPO and for the alkaline hydrolysis of p-nitrophenyl acetate are analyzed quantitatively within the framework of ion exchange in micellar solutions. This analysis points to several potential complexities in the interpretation of reactivity patterns in buffered micellar solutions and leads to criteria for appropriate buffering conditions.
Journal of Physical …, 2012
We studied the cleave of bis(p-nitrophenyl) phosphate (BNPP) over a pH range of 7.0-12.0 in the presence of cationic micelles of cetyldiethylethanolammonium bromide, cetyldimethylethanolammonium bromide, cetylpyridinium bromide, cetyltrimethylammonium bromide, and cetylpyridinium chloride by using different a-nucleophiles, viz acetohydroxamate, benzohydroxamate, salicylhydroxamate, butane-2,3-dione monooximate, and a-benzoin oximate ions. With the use of a-nucleophiles in cationic micellar media, the hydrolytic cleavage of BNPP was found to be approximately 10 5 -fold faster than its spontaneous hydrolysis. All reactions followed pseudo-first-order kinetics. The effect of various concentrations of cationic micelles for the reaction of BNPP and a-nucleophiles has been studied. The variation of k obs values of the reactions depends on the micellar structure, that is, head groups, hydrophobic tail length, and counter ion.